1. Field of the Invention
The invention relates to navigational guidance for mobile users, using Global Navigation Satellite System(s) (GNSSs).
2. Background Art
GNSSs include the Global Positioning System (GPS) developed by the United States Air Force, Global Navigation Satellite System (GLONASS) developed in Russia, which is under needed improvements, Galileo under development in Europe, and possibly other systems such as Compass, currently under consideration in China. However, GPS has been the only complete system and has been widely used in aviation for a number of years. Among these other systems, the implementation of Galileo is promising in the near future. When Galileo becomes fully operational, GPS and Galileo may be used together to improve the navigational performance of mobile users anywhere in the world.
As known in the art, GPS, as will Galileo, provides highly accurate position and velocity information and precise time continuously to an unlimited number of users throughout the world. However, GPS alone, without any other augmentation, lacks a capability known as integrity. That is, if a satellite fault occurs, GPS cannot inform the user of the fault in a timely manner. The integrity issue is critical for aviation because lack of this capability can directly lead to a situation in which safety may be compromised. One important method to provide the integrity for GPS in civil aviation applications is known in the art as Receiver Autonomous Integrity Monitoring (RAIM).
A GPS user receiver only requires four satellites for its position determination; however, five or more satellites are typically visible from any location, thus making redundant GPS measurements available. If redundant measurements are available (i.e., if five or more satellites are visible), a consistency check may be performed among the different satellite range measurements to determine if any of the satellites are faulty with an unusually large ranging error. The idea is that if no satellite is faulty (i.e., has an unusually large ranging error), it is likely that the measurements are consistent. If any one of the satellites is faulty, having an unusually large ranging error, the measurements would not be consistent with each other.
RAIM is essentially an intuitive formulation and can be used for fault detection in navigation applications. The degree with which RAIM detects faults depends on the user-to-satellite geometry formed by visible satellites. RAIM performance is dependent upon position error not exceeding levels for safe operation. RAIM performance is also a function of Pr{Hazardously Misleading Information (HMI)}, which is a probability that the position error exceeds the required alert limit without a timely warning to the pilot. The fraction of time in which integrity monitoring can be provided for a given alert limit, satisfying the Pr{HMI} requirements, is called the availability of RAIM. For many mobile navigation applications, especially for air navigation, RAIM availability of many 9's (i.e., 0.999. . . ) is required. However, this requirement is often difficult to achieve for GNSS navigation applications that would provide significant operational benefits. For this reason, effective RAIM methods that provide high availability are always sought after.
RAIM typically refers to a fault detection (FD) function, which determines the presence or absence of a fault that could cause HMI. Sometimes, however, RAIM is used to refer to the fault detection and exclusion (FDE) function as well. Upon detection of a fault by the FD function, the FDE function is activated and attempts to identify and exclude the source (e.g., a satellite) of the HMI. For this reason, while RAIM FD is related to integrity, the FDE function is related to continuity of navigation. An FDE technique is derived by extending a RAIM detection method, as will be briefly described later. However, in general, the user-to-satellite geometry must satisfy much more stringent criteria for the FDE function to be available than for the RAIM detection function.
What is highly desirable, therefore, are RAIM methods that provide improved availability for their respective applications over the existing RAIM methods. The present invention achieves this goal by trading accuracy for integrity.